WASHINGTON — In his first 100 days in power, President Trump has transformed the nation’s highest office in ways both profound and mundane, pushing traditional boundaries, ignoring longstanding protocol and discarding historical precedents as he reshapes the White House in his own image.

But just as Mr. Trump has changed the presidency, advisers and analysts say it has also changed him. Still a mercurial and easily offended provocateur capable of head-spinning gyrations in policy and politics, Mr. Trump nonetheless at times has adapted his approach to both the job and the momentous challenges it entails.

As Washington pauses to evaluate the opening phase of the Trump presidency, the one thing everyone seems to agree on is that, for better or worse, the capital has headed deep into uncharted territory. On almost every one of these first 100 days, Mr. Trump has done or said something that caused presidential historians and seasoned professionals inside the Beltway to use the phrase “never before.”

He has assumed even more power for the presidency, expanding President Barack Obama’s use of executive orders to offset the inability to pass major legislation and making it more independent of the Washington establishment. He has been more aggressive than any other president in using his authority to undo his predecessor’s legacy, particularly on trade, business regulation and the environment. And he has dominated the national conversation perhaps more thoroughly than any president in a generation.

At the same time, he has cast off conventions that constrained others in his office. He has retained his business interests, which he implicitly cultivates with regular visits to his properties. He has been both more and less transparent than other presidents, shielding his tax returns and White House visitor logs from public scrutiny while appearing to leave few thoughts unexpressed, no matter how incendiary or inaccurate. And he has turned the White House into a family-run enterprise featuring reality-show-style, “who will be thrown off the island?” intrigue.

“His first 100 days is a reflection of how much the presidency has changed,” said Janet Mullins Grissom, a top official in President George Bush’s White House and State Department. “The biggest difference between President Trump and his predecessors is that he is the first president in my political lifetime who comes to the office unbeholden to any special interest for his electoral success, thus immune to typical political pressures.”

In effect, she said, that compensates for a victory he secured in the Electoral College without winning the popular vote. “That gives him as much leverage as someone who won with landslide numbers — and the freedom to govern his way,” she said. “And his voters love him for it.”

Where Washington veterans fret about deviations from past norms, Mr. Trump’s supporters see a president willing to shake things up. Where Washington cares about decorum and process, they want a president fighting for them against entrenched powers.

Yet the crockery-breaking leader has shown signs of evolving. The president operating on Day 100 is not the same as the one who took office in January, when he was determined to make nice with Russia, make trouble for China and make war on elites.

By his own account, Mr. Trump has discovered how much more complicated issues like health care and North Korea are than he realized, and he has cast off some of his most radical campaign promises after learning more about the issues.

“I’m more inclined to say the presidency has changed Trump rather than Trump changed the presidency,” said H.W. Brands, a University of Texas professor who has written biographies of multiple presidents, including Ronald Reagan and both Roosevelts. “He has moderated or reversed himself on most of the positions he took as a candidate. Reality has set in, as it does with every new president.”

All the more so for the first president in American history who had never spent a day in government or the military, and surrounded himself with top advisers who had not either. Although Mr. Trump assumed that his experience in business and entertainment would translate to the White House, he has found out otherwise.

In a separate interview with Reuters, he said: “This is more work than in my previous life. I thought it would be easier.”

Mr. Trump arrived at the White House unimpressed by conventions that governed the presidency. At first, he blew off the idea of receiving intelligence briefings every day because he was “a smart person” and did not need to hear “the same thing every day.” He telephoned foreign leaders during the transition without consulting or even informing government experts on those countries.

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A whirlwind tour of the triumphs and setbacks that marked the first 100 days of Donald J. Trump’s presidency.Published OnApril 27, 2017CreditCreditStephen Crowley/The New York Times

He badgered specific companies on Twitter about moving jobs overseas and called in the chief executive of Lockheed Martin to complain about the cost of the F-35 fighter jet, never mind that presidents typically do not involve themselves in the affairs of individual companies or directly negotiate federal contracts.

Mr. Trump likewise has gleefully taken credit on days that stocks have risen and publicly commented on the strength of the dollar, which presidents generally do not do either, both because it might be viewed as unseemly interference in the markets and because it invites blame when they have a bad day.

His boastfulness knows few bounds. “I truly believe that the first 100 days of my administration has been just about the most successful in our country’s history,” he said in his weekly address on Friday.

His Twitter account, of course, has been the vehicle for all sorts of outbursts that defy tradition, often fueled by the latest segment on Fox News. Presidents rarely taunt reality-show hosts about poor ratings, complain about late-night television comedy skits, berate judges or members of their own party who defy them, trash talk Hollywood stars and Sweden, declare the “fake news” media to be “the enemy of the American people” or accuse the last president of illegally wiretapping them without any proof.

David Gergen, a White House aide to four presidents, including Reagan, noted that Franklin D. Roosevelt talked about the “moral leadership” of the presidency. “Unfortunately, we have lost sight of that vision in recent years, and it has almost disappeared during the first 100 days of the Trump administration,” Mr. Gergen said.

Another change to the presidency involves Mr. Trump’s refusal to release his tax returns — a practice of presidents for 40 years — and his continued ownership of a vast business empire that includes properties both overseas and blocks from the White House. “He has overstepped the ethics limits that have bound all other presidents for decades,” said Norman L. Eisen, a chief White House ethics adviser under Mr. Obama.

Beyond that, Mr. Trump has been slow to create a structure like those in past administrations. Orders and memos have not always been reviewed by all relevant officials. Meetings are not always attended by key aides who are leery of leaving the president’s side. “The notion of a chain of command is gone,” said David F. Gordon, the State Department director of policy planning under President George W. Bush.

But if the presidency had grown somewhat stale under the old norms as its occupants increasingly stuck to carefully crafted talking points and avoided spontaneity, Mr. Trump has brought back a certain authenticity and willingness to engage. His frequent news conferences and interviews can be bracingly candid, uninhibited, even raw. He leaves little mystery about what is on his mind.

“The 2016 election wasn’t a delicate request to challenge existing traditions; it was a demand that our next president do things different,” said Jason Miller, a top adviser to Mr. Trump during the campaign. “And while the professional political class struggles to understand what has happened to their hold on power, supporters of President Trump — the forgotten men and women he referenced in his Inaugural Address — love the change they’re seeing.”

Presumably Mr. Trump will remain impulsive and even impetuous, but he has also been open to advice. He was talked out of lifting sanctions on Russia, moving the American embassy to Jerusalem, abandoning the “one China” policy, tearing up the Iran nuclear agreement, reversing the diplomatic opening to Cuba, closing the Export-Import Bank, declaring China a currency manipulator and, in recent days, terminating the North American Free Trade Agreement. He may still do some or all of these, but by waiting, he has the opportunity to lay the groundwork rather than act precipitously.

He now receives his intelligence briefings most days. And aides said they had noticed signs of growth in office, pointing to his decision to strike Syria after it used chemical weapons on civilians and his private efforts to persuade Egypt to release an imprisoned American aid worker. Both cases showed that Mr. Trump “has absorbed the responsibilities of the office and the impact of the decisions he makes,” said a White House official, speaking on the condition of anonymity to discuss the boss.

Even if Mr. Trump adapts, though, the larger question is whether the institution will ever be the same. Future presidents may feel freer to make unfounded statements, withhold tax returns or keep private business interests without fear of political penalty. Taboos once broken no longer seem inviolable.

Still, Mayor Rahm Emanuel of Chicago, a senior adviser to President Bill Clinton and chief of staff for Mr. Obama, said there might be a backlash once Mr. Trump leaves office. “After Trump, there will be a collective desire to return to tradition,” he predicted. “Whoever comes next will be the anti-Trump in style and character. That’s how it works.”

Karl Rove, the senior adviser to the younger Mr. Bush, agreed. “President Trump will make it difficult for future presidents to step back from the use of social media,” he said, “but it’s very likely the next administration will be more restrained and less personal.” The next president, he added, will probably deploy social media as a premeditated strategy. “It will be part of a plan, not a method of catharsis.”

Meena Bose, the director of the Peter S. Kalikow Center for the Study of the American Presidency at Hofstra University, said Mr. Trump’s presidency so far seemed unlike almost any other, except perhaps Andrew Jackson’s. She noted that Jackson was seen as erratic at the time but was later evaluated by historians as a near-great president.

“Might the Trump presidency be viewed similarly someday?” she asked. “Difficult to see at the 100-day mark, but that is an artificial measurement, with so much of the presidency still to come.”